Micro-satellite for Astrometry and Photometry of Gravitational Wave
Sources
S.Harita,1 T.Ozawa,1 T.Yoshii,1 Y.Yatsu,1 N.Kawai,1 T.Tawara,2 S.Matunaga,2N.Tominaga,3 S.Kulkarni,4 P.Bilgi,4 T.Sakamoto5 and N.Vasquez6
1Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama Meguro Tokyo
Department of Mechano-Aerospace Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama Meguro Tokyo
3Department of Physics, Faculty of Science and Engineering, Konan University, Okamoto, Kobe
4Division of Astronomy, Mathematics & Physics, 105-24, California Institute of Technology, Pasadena, CA 91125
5Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara
6Department of Phisics, Escuela Politécnica Nacional, Ladron de Guevara E11-253, Quito 170517
E-mail(SH): harita@hp.phys.titech.ac.jp

2Abstract
We propose a micro-satellite for surveying EM counterparts of gravitational wave sources and the timedomain astronomy in ultraviolet. Several theoretical study predicts NS-NS mergers may produce strong
UV emission prior to the optical and IR emission. If that is the case wide-field UV monitor is usefulfor detection and position determination of EM counterparts. For this purpose we designed a ϕ200mm
reflector and a satellite bus for this mission. The expected limiting magnitude is ∼22 magAB in the200−300 nm band for 300s exposure. Cooperate with the agile satellite bus system, this satellite system
can survey 100 deg2 with a cadence of 2 cycle h−1 . With an assuming of UV emission from free-neutron
decay, more than one NS-NS merger can be detected with the system.
Key words: Ultraviolet, Time-domain astronomy, Gravitational Wave, Micro-satellite

1. Introduction
In 2015, the first gravitational wave (GW) event in human history was finally detected. The astronomers are
now aiming for detecting the electromagnetic counterparts of GW events. The technical barrier to be overcome is the large error circle of the GW telescopes, typically larger than ∼100 deg2 . Therefore the prompt andaccurate position determination is the necessary step towards the multi-messenger astronomy in the GW-era.
2. Target Profile
The first GW event seemed to be generated by a binary black-hole that believed not to produce any radiation. Therefore we expect neutron star (NS)-NS mergers
which produce r-nuclei and these radioactive ejecta result in the kilo-novae. The nature of the kilo-novae isstill unclear and numerous theoretical models are proposed. Part of the claimed that free neutrons in theouter edge of ejecta produce ultraviolet photons due tothe beta-decay(Brian D. Metzger et al. 2015).
In this scenario, as the emitting region is exposed theluminosity is should be higher and prior to optical/IRphotons which experienced scatterings and absorption

Table 1. Mission Requirements
ParameterBand.......
Detection limit.......
Time span.......
Survey area.......
Cadence.......
Delay of Alerts.......
Detection rate.......

Requirement200∼300 nm≥22 magAB∼ 1 hour≥ 100 degree2≥ 2 cycle hr−1
≤ 30 min≥ 1 event yr−1

in the optical thick r-nuclei. Fig. 1 shows the simulated
light curves in various colors of a kilo-novae at 200 Mpc
from the earth. The bluer the peak flux is higher, 22magAB in U-band. The rise time is about 1 hour whichis shorter than that of the typical kilonovae model (noneutron heating model) by factor of 1/10.
If that is the case, ultraviolet which is brighter andfaster than the other colors can be the best way to detectand determine the position of the GW source. Moreoverthe its light curve has crucial information for constraint
the physical state of the condensed matter in neutronstar, i.e. the equation of states, the r-process nucleo
synthesis.

Table 2. Specifications of the Detector System

Parameter
Diameter.......
Focal length.......
Field of view.......
UV Transmittance.......
PSF.......
CCD size.......
pixel number.......
Quantum eﬃciency.......
Dark current.......

Value200 mm430 mm
17 deg2
70 %15 arcsec (FWHM)
31 mm × 31 mm
2064 × 2064 pix
60 ∼ 80 %@200∼300 nm
8.3 e− s−1 pix−1 @ −30◦ CTable 3. System Requirements

Fig. 1. Simulated light curves a NS-NS merger from Brian D. Metzger
et al. (2015).

3. Mission Requirements & Design
Based on the target profile shown in the previous section, the mission requirements are summarized in table
1. It is diﬃcult to transfer all the data to the groundinstantly, we must analyze the raw-data and detect transient source on-board. In this case, two images taken at
the diﬀerent time epoch is required to compare the luminosity of the candidates. If we start the observation justafter the GW detection, we must take more than two
images before the maximum. This result in the lower
limit ofParameter
Required
Attitude stability
15 arcsec / 10s
Slew speed≥1 deg s−1
Visible Area0∼ 40deg
Power supply
96 W
Data rate
30 MB day−1†: angle from the anti-sun direction.

Designed>15arcsec /10s
8 deg s−10∼40 deg†
120 W @ EOL
36 MB/day−1such as the solar array paddles. For this purpose Hibari
has four 400mm x 400mm solar array paddles mounted
on the end of 400mm long arms and these arms aredriven by motors via reducing gears. Remarkably thismethod enables rapid maneuver faster than 10 deg s− 1
whilst maintaining attitude stability higher than 15 arcsec s− 1. In addition, it does not requires additional spaceand mass for the actuator as is for control moment gyroscopes (CMGs) but only motors and small electrical
2 −1[Num of tiling]×[FoV]×[Cadence] ≥ 200 deg hr .(1)power. Therefore this method can be valuable especially
While the payload size is strictly limited by the piggy- for the sophisticated missions on micro-satellites withback regulation of the H-IIA launching vehicle. The critical limitations in power, weight and space.
Thanks to the VSAC which eﬀectively utilizes the limmaximum diameter of the telescope is ϕ200 mm thatitedresource, the satellite bus is fall within the criterion
requires the minimum exposure time of 300 s to detectof
50×50×50
cm3 . In addition the power consumption
22 magAB . Therefore the field of view of the telescope
system should be larger than 17 deg2 . To satisfy the re- of the satellite is below 50 W, which is about 50 % of ourquirements and that can be put into the satellite bus, we previous 50 kg-satellite with CMGs. The science mission
also requires real time RF communication for followingemployed the Riccaldi-Honders optical system.
As the ultraviolet imager to be coupled with the tele- up GW events and broadcasting detection alerts to thescope, we employed a back-illuminated CCD developed ground. For this purpose we employ the Iridium netby Caltech for UV survey missions. The CCD has large work. This shrinks the delay of both up/down-links
physical format of 31 × 31 mm2 and high Q.E. ≥ 80%. within 30min on average. With this satellite, more thanThat fulfill the our mission requirements. The specifica- one UV counterparts of GW sources and a few tens oftions of the detector systems are summarized in Table core-collapse supernovae and the other unknown phenomena can be detected per year.
2.
4. System DesignThe above mission strategy requires both high-speed andquite stable attitude control system. For this mission weare developing a novel attitude control method, namelyVariable Structure Attitude Control; VSAC (KyosukeTawara, and Saburo Matunaga 2015) which utilizes
the anti-torque generated by swinging the appendages,

References
Brian D et al.“Neutron-powered precursors ofkilonovae”,MNRAS, 446, 1115-1120, 2015.
Kyosuke Tawara, and Saburo Matunaga, “On Attitude Control of Microsatellite Using Shape Variable Elements,” The 24th Workshop on JAXA: Astrodynamics
and Flight Mechanics, Sagamihara, July 2015.